An experimental weight-loss program designed to change eating and exercise habits for the entire family can help obese children lose weight and keep it off for up to 10 years, according to a report in the current issue of the Journal of the American Medical Association.

Extra pounds are hard to lose for both adults and children, and few diet programs prevent the weight from coming back once the dieting regimen is stopped. The behavior-changing approach developed at the University of Pittsburgh is the first to show some small but long-term weight loss in obese children, said Leonard H. Epstein, a psychologist who is the study director.

"Obesity is a family problem," he said. "If you have a family environment where you have heavy parents and heavy kids, you {probably} have a family that overeats, eats the wrong food and is sedentary."

For a diet to work, parents must be involved because children don't buy or prepare food or create the basic family environment. Although genetic inheritance plays a role in a person's size, eating behavior is a key factor in obesity.

While the definition is somewhat arbitrary, most experts consider a person obese if the accumulation of body fat exceeds standard height-weight tables for age and sex by 20 percent. Obesity is considered mild when excess body fat exceeds height-weight tables by 20 to 40 percent, moderate when it is 41 to 100 percent, and severe when it is more than 100 percent.

The Pittsburgh study involved 76 families in which there was an obese child and at least one obese parent. To change the family's eating pattern, the Pittsburgh researchers divided the families into three groups and designed a program of eight weekly meetings and four monthly meetings where participants were taught about diet and exercise. All three groups received the same information, but not the same rewards for losing weight.

In group one, rewards were given only if both parent and child lost weight. In group two, they were rewarded if only the child lost weight. In the third group, they were rewarded merely for coming to the meetings. Rewards included cash rebates for the parents and special activities for the child, such as a trip to the zoo.

The group in which both the parent and the child received a reward if they both lost weight did the best, apparently because of the parent's personal involvement in the diet. The average weight of the children in that group dropped from 42 percent overweight to 35 percent overweight.

Children in the other two groups actually became more obese. Those in the second group gained 5 percent more weight, going from being 44 percent overweight to being nearly 50 percent overweight. The last group went from 46 percent overweight to 65 percent overweight, a 19 percent increase.

While the children in the first group became less obese, most of them did not get down to the normal weight for their age and height. Nevertheless, reversing the trend toward increasing weight gain is seen as an impressive achievement by many diet researchers.

Successful weight-loss programs are in demand because of the national obsession with thinness, especially for adolescent girls. Many dieters may find themselves bouncing in and out of regimens that do not work and can even be harmful.

Beyond the thinness craze, however, is the growing realization that obesity is tied to a number of illnesses, even in children. For example, half of all children with high blood pressure, and one third of children with high cholesterol levels, are obese, said William H. Dietz, a pediatrician at the New England Medical Center in Boston. "And the psychological impacts are not to be dismissed," he said.

Still, the most severe diseases linked to obesity don't occur until midlife. They include heart disease, uterine and breast cancers and diabetes.

Researchers are focusing new attention on reducing obesity in childhood as a way to reduce disease later in life. "It is not good to be fat," said Albert J. Stunkard, a psychiatrist and weight researcher at the University of Pennsylvania who co-authored an editorial on the dieting study. "Fat kids grow up to be fat adults."

In a study of eighth-graders from Hagerstown, Md., who were followed into adulthood, Stunkard found that an obese eighth-grader had a 4-in-1 chance of being fat at age 30, and that if the child had not lost weight by the end of adolescence, the odds jumped to 28-to-1.

A large study at Wright State University in Dayton that has been tracking thousands of family members over generations since 1929 found that children who are obese at age 2 to 3, often end up as obese adults.

"We used to say a fat baby is a healthy baby," said Gilman Grave of the National Center for Child Health and Human Development. "Probably the opposite is true."